The present invention relates to a method for manufacturing simultaneously ground type bearing body suitable for use in a table saddle of a machine tool, slide surfaces in a machining center and other transfer devices, such as robots and the like, which are adapted to make reciprocatory sliding motion while carrying heavy loads. The invention also pertains to an unlimited sliding bearing unit employing the simultaneously ground type bearng body.
In a typical conventional unlimited sliding bearing unit, as shown in FIG. 19, a pair of recesses 105, 106 are formed on the right and left (as viewed in FIG. 19) of a central cavity 102 of a bearing body 101 by means of broaching. In addition, two ball rolling surfaces 107, 108 are formed on both side surfaces of the recess 105, while two ball rolling surfaces 109, 110 are formed on both side surfaces of the recess 106. Then, the ball rolling surfaces 107, 108, 109, 110 are hardened to produce the bearing body 101. Since the ball rolling surfaces 107, 108, 109, 110 may be distorted owing to heating in the hardening, however, the ball rolling surfaces are generally finished by means of grinding employing a grind stone to obtain a required demensional accuracy.
The conventional grinding for finish is such that, as shown in FIG. 19, the ball rolling surfaces 108, 110 closer to the upper sides of the right and left skirts 103, 104 are first ground successively by means of a grinding wheel G which revolves with its axis of rotation placed vertically, and then, the ball rolling surfaces 107, 109 closer to the lower sides of the right and left skirts 103, 104 are successively ground by means of a grinding wheel G which revolves with its axis of rotation placed horizontally. The prior art, however, is not suitable for mass production, since four steps are required for the grinding operation. Moreover, owing to the individual grinding for each of the four ball rolling surfaces, it is difficult to uniformly grind all the ball rolling surfaces to close tolerances. In consequence, in order to obtain required dimensions with high accuracy, an impractically long time is needed for the grinding operation, which constitutes one of factors in a high cost. If a required dimensional accuracy cannot satisfactorily be obtained, e.g., if the infeed of the grinding wheel is excessive, there will be play between the ball rolling surfaces on the bearing body and the corresponding ball rolling surfaces formed on the associated guide rail when balls are recirculated through the passageways defined by these ball rolling surfaces, resulting in various problems, such as a poor torque transmission, the generation of noise and the inclination of the bearing body. If the infeed of the grinding wheel is insufficient, on the other hand, it becomes difficult to insert balls into the space defined between the ball rolling surfaces on the bearing body and the corresponding ball rolling surfaces on the guide rail. In addition, after the insertion of the balls, the movement of the balls becomes stiff due to an increase in resistance to sliding, which makes it impossible to obtain a smooth traveling of the bearing unit disadvantageously. Summary of the Invention:
Accordingly, it is an object of the invention to provide a method for manufacturing a simultaneously ground type bearing body which can be mass-produced at low cost and has ball rolling surfaces finished with high accuracy, by not only simultaneously grinding two opposite ball rolling surfaces disposed on either one of the sides of the central cavity but also simultaneously grinding a total of four ball rolling surfaces disposed on both the sides of the central cavity.
It is another object of the invention to provide a method for manufacturing a bearing unit improved to increase its torque transmitting capacity as well as perform a noiseless, smooth traveling, by incorporating the above-mentioned simultaneously ground type bearing body as a constituent element thereof.
Above and other objects, features and advantages of the invention will become apparent from the following description taken in connection with the accompanying drawings.